This invention relates to ceramic coatings of the type used to protect components exposed to high temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a multilayer ceramic coating system that is built up on a substrate with one or more ceramic-containing tapes to produce an outermost ceramic layer with a smooth outer surface and at least one interior ceramic layer whose mechanical and physical properties are selected to provide a transition between the substrate and outer ceramic layer.
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles.
Coating systems capable of satisfying the above requirements typically include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component. Metal oxides, such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure. Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant alloy such as MCrAlY (where M is iron, cobalt and/or nickel). Aluminide coatings are distinguished from MCrAlY coatings, in that the former are intermetallics while the latter are metallic solid solutions.
While coating systems of the type described above are widely employed, the requirement that the coating system remain adherent throughout many heating and cooling cycles is particularly demanding because the coefficient of thermal expansion (CTE) of ceramic materials is significantly lower than those of the superalloys typically used to form turbine engine components. Such differences in CTE, in combination with oxidation of the underlying bond coat or substrate, eventually lead to spallation of the coating system.
An additional desired characteristic for a coating system of a gas turbine engine component is for the outermost surface of the coating system to be extremely smooth in order to promote the aerodynamics of the component surface. While relatively smooth ceramic coatings can be produced with spray methods -such as those noted above, particularly with PVD techniques, smoother surface finishes would be desirable. In general, deposition techniques noted above tend to produce ceramic coatings that are relatively porous, which is advantageous in terms of achieving a low coefficient of thermal conduction. However, porosity promotes surface roughness-ceramic coatings deposited by PVD generally have surface roughnesses of about 60 xcexcinch (about 1.5 xcexcm) Ra and higher, and those deposited by APS and LPPS typically have surface roughnesses of about 260 to 400 xcexcinch (about 6.6 to 10.2 xcexcm) Ra. Ceramic coatings deposited by conventional spray methods on components with complex geometries are further prone to such surface flaws as shadowing effects (thin or beaded regions caused by partial masking due to part shape) and slumping (thicker regions formed by movement of material to low portions of a part due to gravity).
In view of the above, it can be appreciated that there is an ongoing demand for gas turbine engine components that can be produced with adherent thermal barrier coatings having exterior surfaces that are denser and smoother for improved aerodynamical performance.
The present invention generally provides a method for forming a ceramic coating on a substrate that requires thermal protection from a hostile thermal environment, such as the turbine, combustor and augmentor sections of a gas turbine engine. The method is particularly directed to producing a multilayer ceramic coating system whose outer surface is dense and smooth for improved aerodynamics, while the remainder of the coating system is more porous to promote the ability of the coating system to thermally insulate the component and provide a more uniform transition of mechanical and physical properties between the component surface and the dense surface of the coating system, thereby promoting the spallation resistance of the coating system.
The method of this invention generally entails forming at least two tape compositions that contain ceramic particles dispersed in an organic constituent, such as a binder and/or plasticizer. In one embodiment, a tape is formed of the first composition and then applied to the substrate to form a n innermost tape layer on the substrate, after which at least one additional tape is formed of the second composition and then applied to form an outermost tape layer. The tapes are then processed (cured then sintered) to form innermost and outermost ceramic layers, respectively, on the substrate (along with any intermediate tapes that can be applied between the innermost and outermost tape layers to form intermediate ceramic layers upon curing and sintering). Alternatively, a single multilayer tape can be formed to contain both tape compositions, with the first tape composition forming a lower layer of the tape and the second composition forming the upper layer of the tape. After applying the tape to a substrate and then curing and sintering, the first and second compositions form innermost and outermost ceramic layers, respectively, on the substrate. This embodiment also permits the use of intermediate compositions to form intermediate layers within. the tape and, subsequently, intermediate ceramic layers on the substrate.
According to the invention, the tape or tapes are formed and processed such that the innermost ceramic layer has submicron voids to provide a desired level of porosity, while the outermost ceramic layer is thinner, smoother and less porous than the innermost ceramic layer. As a result, this invention provides a multilayer ceramic coating system having a sintered inner ceramic layer whose porosity can be tailored to yield mechanical and physical properties that promote the adhesion of the coating system, and a denser and smoother sintered outer ceramic layer whose density and surface roughness can be tailored to promote the aerodynamic performance of the component on which the coating system is formed. Additional ceramic layers between the inner and outer layers can be formed if desired to further grade the transition between the substrate and coating surface. The result is a coating system that, as a whole, has a smooth aerodynamic surface, a low coefficient of thermal conductivity, and resists spalling throughout many heating and cooling cycles.
Other objects and advantages of this invention will be better appreciated from the following detailed description.